1. Field of the Invention
The present invention relates to a tool holder which grasps and holds a cutting tool such as a drill in a collet chuck by a slotted taper collet and a locknut, and more particularly, it relates to a tool holder in which a contact part between a locknut and a taper collet is coupled via a bearing washer.
2. Description of the Prior Art
In terms of a conventional tool holder of this type, in order to grasp and hold a cutting tool such as a drill in a collet chuck body by a slotted taper collet and a locknut, a coupling groove is formed in a ring shape around the entire outer circumference of a cutting tool inserting-side end portion of the taper collet, a convex thread portion to be engaged with the coupling groove is provided in a ring shape on the inner circumference of the locknut, and this convex thread portion is engaged with the coupling groove, whereby the taper collet is coupled to the locknut so as to be relatively rotatable.
Next, the taper collet in a coupled condition is inserted into a tapered hole provided on the inner circumference of the front-end portion of the collet chuck body and the locknut is screwed with a male screw portion provided on the outer circumference of the front-end portion of the collet chuck, whereby the taper collet is temporarily attached by insertion in the collet chuck body. Then, by tightening the locknut after inserting a shank portion of the cutting tool into a tool insertion hole, the taper collet is pushed into the tapered hole of the collet chuck body and is reduced in its diameter, whereby the cutting tool is held in the collet chuck body (See Non-patent literature 1, for example).
[Non-Patent Literature 1]
“Technical skill box 19: Usage of working tools, collet chuck structure” published by Taiga Shuppan, Jan. 1, 1975, pp. 22-23.
However, in such a conventional tool holder as described above, since the convex thread portion of the locknut is directly engaged with the coupling groove of the taper collet, when the tapered portion of the taper collet is pushed into the tapered hole of the collet chuck body by screwing the locknut with the male screw portion of the collet chuck body and tightening the same, press force to the tapered collet is biased due to surface roughness and friction, etc., between the contact surfaces of the mutually engaged ring-like coupling groove and ring-like convex thread portion, and moreover, torsional force due to a rotation of the locknut is transmitted to the taper collet. Due to this biased press force, the tapered portion of the taper collet may be pushed into the tapered hole of the collet chuck body in an inclining manner, and this phenomenon becomes noticeable as the collet becomes smaller in size.
In addition, if the taper collet is chucked in the collet chuck body in an inclined manner, a rotation run-out occurs at the cutting tool grasped and held in the taper collet, therein exists a problem such that a harmful influence is exerted to the workpiece cutting work.
In addition, as a result of the torsional force transmission to the taper collet, the taper collet grabs and supports the cutting tool in a twisted condition, therein exists a problem such that chucking of the cutting tool is hindered.